Promoters for radiation induced cross-linking in polymer substances

ABSTRACT

Process for the production of vulcanizates of diolefin rubbers by irradiation of compositions containing such rubbers utilizing polyhydrosulfides such as 1,10-dimercaptodecane as direct promoters. Indirect promoters may also be used to lower the total dose requirements.

This is a continuation of application Ser. No. 555,110, filed Mar. 4,1975, which is a continuation of application Ser. No. 395,177 filedSept. 7, 1973 now abandoned, which is a divisional of application Ser.No. 148,883 filed June 1, 1971, now U.S. Pat. No. 3,843,502.

BACKGROUND OF INVENTION

This invention relates to the irradiation vulcanization of vulcanizablerubbers utilizing polyhydrosulfides as cross-linking agents. Moreparticularly, it relates to new and improved processes for thevulcanization of diolefin rubbers in the presence of polyhydrosulfidesusing high energy ionizing radiation. It relates also to the novelvulcanizable compositions and to the vulcanizates produced.

Efforts are continually being made to improve the properties of naturaland synthetic rubbers. New rubbers are produced containing a variety ofaccelerators, antioxidants, vulcanizing agents and other additives.Attempts have been made to utilize high energy particle irradiation toimprove the properties of various rubbers and other polymers both withand without additives. These attempts have not been successful becausesome rubbers degrade when subjected to irradiation and others, althoughsomewhat improved in properties, require such a high dose to acquireeven minimum beneficial results that the process is generally noteconomically attractive.

THE INVENTION

A process has now been discovered for the vulcanization of uncuredrubbers whether obtained by solution or emulsion polymerization toproduce vulcanizates of improved properties by radiation vulcanization,and to do so at dose levels consistent with good economic practice.

The process of the invention is particularly useful for natural andsynthetic diolefin rubbers, a well known class of rubbers including, forexample, polybutadienes, polyisoprenes, various copolymers of conjugateddiolefins and vinyl compounds such as SBR (copolymer of butadiene andstyrene) and NBR (copolymer of butadiene and acrylonitrile), copolymersof butadiene or isoprene with alpha-methylstyrene, ring-substitutedstyrenes, chloromethyl styrene, etc., butadiene-isoprene copolymer,isobutylene-isoprene, isobutylene-cyclopentadiene, vinyl pyridines,ethylene-propylene terpolymers, etc. The invention is also applicable torubber blends such as natural rubber and SBR, natural rubber andpolybutadiene, SBR and polybutadiene. These blends may additionallycontain butyl or chlorobutyl rubbers. The application of radiationtechniques to these last mentioned rubbers is a particular feature ofthis invention which will be discussed in more detail hereinafter.

Any of a wide variety of polyhydrosulfides which have two or moresulfhydryl groups and are miscible with the unvulcanized rubbers areuseful in the practice of this invention. Suitably, the polyhydrosulfidewill have a molecular weight in the range from about 94 to 500.Typically useful compounds include 1,2-ethane dithiol,1,3-propanedithiol, tetra methylene dithiol, hexamethylene dithiol, thecyclohexane dithiols, 2,3-dimercaptopropanol-1, α,α-dimercaptoxylene,ortho-, meta- and paraphenylene dithiols, 1,10-dimercaptodecane,ethylene glycol dimercaptoacetate, dipentene dimercaptan,2,2-diphenyl-1,3-propanediol dithioglycolate and trimethylolpropanetrithioglycolate. Ethylene glycol dimercaptoacetate andtrimethylolpropane trithioglycolate are especially preferred speciesbecause they have good reactivity at relatively low doses, their odor isnot offensive, and they produce excellent products.

Other useful species include those in which the organic polyhydrosulfideis an aryl or alkyl polymercaptan having from about 2 to 20 carbon atomsper molecule. Especially preferred are alkyl dimercaptans having two toten carbon atoms per molecule.

It will be noted that the polyhydrosulfides utilized in the inventionmay have other functional groups present, including, for example,carboxyl, nitrile, hydroxyl, and other functional groups which do notinterfere with the reaction. This is a special advantage of theinvention since it affords the opportunity of producing a wide varietyof vulcanized rubbers having chemical or physical properties which makethem useful for special purposes. For example, automotive tiresconstructed of rubbers prepared in accordance with the inventionutilizing polyhydrosulfides with carboxyl groups on the molecule as thecross-linking or curing agent have improved anti-skidding properties,especially on wet surfaces. Additionally, the presence of otherfunctional groups offers the opportunity for further chemical reactionof the vulcanized rubber.

The polyhydrosulfides utilized in this invention appear to function ascross-linking agents, that is the vulcanization is effected by theformation of crosslinks joining adjacent linear chains of theunvulcanized rubber together. In this sense they are direct promoters ofvulcanization. The products formed therefore, are characterized by thepresence of both carbon-carbon and carbon-sulfur crosslinks.

In an especially preferred aspect of this invention certain selectedindirect promoters are added to the composition to be vulcanized. Paradichlorobenzene is a preferred indirect promoter. Others include ortho-,and meta-dichlorobenzene, ortho-, meta- and paradibromobenzene, ortho-,meta- and para difluorobenzene, bromobenzene, chlorobenzene,chlorotoluenes; mono- or poly-chlorinated biphenyls,dichloronaphthalenes, chloroform, carbon tetrachloride, carbontetrabromide, hexachloroethane, 1,2-dichloro-4-t-butylbenzene, t-butylchloride, cyclohexyl bromide, α-chlorotoluene, and α,α'-dichloroxylene.

It has been found that the crosslink density which may be obtainedutilizing both direct and indirect promoters is greater than would bepredicted on the basis of the crosslink density which can be attainedusing either one of them alone. In other words, a synergistic effectwhich was totally unexpected has been observed. This effect will beillustrated in the examples. The use of both types of promoters permitsa decrease in the total irradiation dose required to achieve theimproved properties. Alternatively, the concentration of direct promotercan be lowered without adversely affecting the results achievable at aspecific dose level. The ultimate result is that the cost of the processis lessened.

Indirect promoters are a class of compounds well known in the field ofradiation. They have been widely employed to improve the effects of highenergy ionizing radiation in the production of various products,particularly cross-linked, high molecular weight polymers andcopolymers. They are generally low molecular weight aliphatic andaromatic compounds containing chlorine, bromine, fluorine or iodine inaddition to carbon and hydrogen up to a total of about ten carbon atoms.

The vulcanizable compositions of this invention comprise uncured,diolefin rubbers having an average molecular weight of from about100,000 to 500,000 containing from about 0.1 to 5%, based on the totalweight, of at least one polyhydrosulfide direct vulcanization promoter.They may also contain from about 0.1 to 5% by weight of an indirectpromoter.

A highly preferred direct promoter is 1,10-dimercaptodecane, and thedichlorobenzenes are excellent indirect promoters. When used incombination, these promoters provide excellent products at relativelylow dose.

The diolefin rubber vulcanizates of the invention comprise diolefinrubbers containing molecular segments, generally linear, which arecross-linked with the internal polyhydrosulfide vulcanization promoters.The average molecular weight of the cross-linked segments is from about100,000 to 500,000.

A most unusual and highly important aspect of this invention is thediscovery that it is applicable to the vulcanization of copolymers ofisoprene and isobutylene containing from about 0.3 to 3 mole percent ofthe former. It is also applicable to the vulcanization of chlorobutylrubbers, which are derivatives of butyl rubber formed by chlorination ofthis substrate. Butyl rubbers have not previously been successfullyconverted to useful products by radiation techniques. In fact, suchattempts as have been made at radiation vulcanization of these rubbershave led to degradation and loss of useful properties rather thanvulcanization and improvement of properties. The synergistic effectdiscussed above has not been observed with chlorobutyl rubber. The rateof vulcanization decreases with the addition of an indirect promoter.

The desired vulcanization can be accomplished in accordance with thisinvention by subjecting the selected composition to an adequate dose ofhigh energy particle irradiation. The technique serves to effectvulcanization and thereby achieve the desired objects. The radiation canbe carried out by passing the composition, suitably in the form of athin layer of from about 0.05 to 0.25 inches in thickness through a beamof high energy electrons under conditions that will give a high energydose of from 0.1 to 15 Mrads. Such a dose is sufficient to effect anydesired degree of vulcanization from a relatively flexible rubberproduct to a hard rubber product. In general, doses of from 1 to 5 Mradsare preferred. Typically the dose rate is from about 0.02 to 5 Mrad/sec.although appreciable variation is possible. Thicker layers of rubber,for example up to about 0.5 inches in thickness, can be treated by twopasses through the beam, first on one side and then on the other.

Various dose rates can be achieved from conventional radiation equipmentby techniques known in the art. For example, the desired amount ofradiation can be obtained using a 300 Kev electron accelerator whichemits a circular beam of electrons from a heated cathode filament. Theelectrodes are guided through a scanning magnet and emerge through athin titanium window which is approximately 1 by 24 inches. A beam powerof 3 Kw is obtained at an operating current of 10 mamps. The dosedelivered can be measured using the well-known blue cellophanetechnique. See Henley and Richman; Anal. Chem. 28, 1580 (1956). Byaltering the beam current, beam diameter and distance to the source,various dose rates can be obtained.

In practicing the invention, the rubber to be vulcanized is compoundedin the usual way which will, of course, vary with the end use of thevulcanized product. If the rubber is to be used in tire construction itmay contain reclaim rubber, oil, vulcanization accelerators,antioxidants, tackifiers and plasticizers, and carbon black. If it isintended for the preparation of hard rubber casing it may containplasticizer, reclaim rubber and hard rubber dust. It is a specialfeature of this invention that the compositions containing thevulcanizable rubber be prepared in accordance with standard proceduresnormally employed for the compounding of rubber compositions intended tobe vulcanized in the usual manner. Polyhydrosulfide is added to thecomposition together with the indirect promoter, if desired, and thecompositions converted to a suitable form for radiation. If one or morepolyhydrosulfides are employed the concentration may vary from about 0.1to 5%, by weight, based on the total weight. The preferred concentrationis from 0.25 to 1.5% since suitable products can be obtained withoutsignificant increase in cost. With rubber blends the concentration ofpromoter will generally be closer to 5% if no indirect promoter is used,e.g. 3.5% to 5%.

Indirect promoters, if employed, will normally be utilized at aconcentration of from about 0.1% to 5% by weight based on the totalweight. By using such indirect promoters it may be possible to operateat a decreased dose level or decreased direct promoter, or both. Theactual amount of each type of promoter utilized therefore, is a matterof choice depending upon the desired result.

Generally speaking, the stress-strain properties of the vulcanizedproduct appear to improve with increasing dose and decreasingconcentration of direct promoter, at least at the lower concentrationsand lower doses.

It is known to polymerize liquid polymers using polyhydrosulfidepromoters. However, these are liquid polymers of relatively lowmolecular weight, whereas the diolefin rubber polymers which are thesubject of this invention are of high molecular weight, i.e., 100,000 to500,000. It has been discovered, as explained above, that theconcentration of polyhydrosulfide utilized to effect vulcanization inaccordance with this invention it very low. Preferred concentrations areof the order of from 0.25 to 1.5% by weight. In fact, concentrationsmuch higher than this, for example in the range of from 1 to 10 molepercent or 2 to 20% by weight, are generally detrimental to the uncuredrubbers to which this invention is applicable since the vulcanizedrubbers produced even at low total doses are no longer pliable andtough, but rather they are hard and brittle. The useful elastomericproperties of the rubber are lost.

A very important commercial advantage of the process of this inventionis that the rate of vulcanization is substantially increased comparedwith conventional processes. This is an important consideration forindustrial operations where many tons of rubber compounds must beprocessed. Such huge quantities can only be processed economically if itcan be done at a high rate of speed. Another advantage is that it can beutilized with both solution and emulsion polymerized rubbers to lowerthe dose requirements for the economic production of useful products.This invention makes it possible for the first time to effect curing orvulcanization of rubber on a commercial scale by radiation techniques.

The following non-limiting examples are given by way of illustrationonly.

EXAMPLES 1-3

The mixtures shown in Tables 1, 2 and 3 were mixed in a Brabenderblender to form a homogeneous mix. The mix was rolled on a mill at 75°C. to form a thin film approximately 25 mils thick. Sections of the filmwere stacked in a platen press and formed into samples 6 inches by 6inches which were about 75 mils thick at a temperature of 100° C. and apressure of 8000 psi on the ram. Irradiation was effected by subjectingthe samples to the electron beam of a 1.5 Mev electron accelerator.

Stereon 700 is a copolymer of butadiene (80%) and styrene (20%) with anumber average molecular weight of about 120,000.

Diene 55 is a polybutadiene with a number average molecular weight ofabout 180,000.

HAF is a high abrasion furnace black.

Arochlor 1242 is a chlorinated biphenyl.

                  Table 1                                                         ______________________________________                                        Stereon 700                                                                            100 parts                                                            HAF       50 parts                                                            Dose           8         5         3                                          Modulus (psi)  1175      1225      1215                                       Tensile (psi)  2664      2925      2535                                       Elongation (%) 600       590       540                                        Trimethylolpropane-                                                            trithioglycolate (phr)                                                                      0         0.5       0.5                                        Arochlor 1242 (phr)                                                                          0         0         3.5                                        ______________________________________                                         These data show that it is possible to lower the radiation dose to achiev     substantially the same ultimate properties by the addition of a direct        promoter and to lower it still more by the further addition of an indirec     promoter.                                                                

                  Table 2                                                         ______________________________________                                        Stereon 700 100 parts                                                         HAF Black    50 parts                                                         p-Dichlorobenzene                                                                          2 parts                                                          Dose (Mrad)    1         1         2                                          Modulus (psi)  overcured 1587      1755                                       Tensile (psi)  "         2550      2850                                       Elongation (%) "         455       435                                        Dimercaptodecane (phr)                                                                       3         1         0.5                                        ______________________________________                                         These data show the improvement in properties of the cured rubber upon        lowering of the concentration of dithiol.                                

                  Table 3                                                         ______________________________________                                        Diene 55    100 parts                                                         HAF Black    50 parts                                                         p-Dichlorobenzene                                                                          2 parts                                                          Dose (Mrads)   0.5       0.5       0.5                                        Modulus (psi)  Overcured 1040      1063                                       Tensile (psi)  "         1340      1785                                       Elongaton (%)  "         350       450                                        Dimercaptodecane (phr)                                                                       3         1         0.5                                        ______________________________________                                         These data show the continued improvement in the properties of the cured      rubber by successive decreases in dithiol concentration. A still lower        concentration than 0.5 phr results in further improved properties.       

EXAMPLE 4

The ingredients listed in Table 4 were compounded and treated as inExamples 1-3 with the listed results.

Chlorobutyl HT 1068 is a chlorinated butyl rubber with a weight averagemolecular weight of about 350,000 to 400,000 a mole percent of isopreneof from 1.1 to 1.7 and a weight percent of chlorine of from 1.1 to 1.3.

                  Table 4                                                         ______________________________________                                        Chlorobutyl HT 1068                                                                          100 parts                                                      HAF Black       50 parts                                                                   Dose (Mrads)                                                                     6        4         5                                          ______________________________________                                        300 % Modulus (psi)                                                                          334       351       369                                        Tensile (psi)  1085      1250      1275                                       Elongation (%) 725       740       725                                        Trimethylol propane                                                            trithioglycolate (phr)                                                                      0         3         3                                          Arochlor 1242 (phr)                                                                          0         0         5.2                                        ______________________________________                                         These data show that chlorobutyl rubber can be vulcanized by radiation an     that the dose can be lowered while attaining substantially the same           properties by the addition of a direct promoter. It will be noted that th     addition of an indirect promoter causes an increase in the required dose.

EXAMPLE 5

A blend containing the following ingredients was compounded and treatedas in the previous examples.

    ______________________________________                                        Ingredients      Parts per Hundred Rubber                                     ______________________________________                                        Stereon 700      50                                                           Diene 35         25                                                           Select A (Natural Rubber)                                                                      25                                                           Flexon 765 Oil   20                                                           HAF Black        60                                                           p-Dichlorobenzene                                                                               2                                                           ______________________________________                                    

Diene 35 is a polybutadiene with a number average molecular weight ofabout 110,000.

Flexon 765 is a naphthenic petroleum oil.

The results are shown in Table 5.

                  Table 5                                                         ______________________________________                                                         Dose (Mrads)                                                                  6        3                                                   ______________________________________                                        300% Modulus (psi) 757        731                                             Tensile (psi)      1967       1871                                            Elongation (%)     550        550                                             1,10-Dimercaptodecane (%)                                                                        0          0.40                                            ______________________________________                                         It is apparent that the addition of the direct promoter halves the            radiation dose necessary to obtain substantially the same properties.    

What is claimed is:
 1. Vulcanizable compositions suitable for theformation of vulcanizates of diolefin rubbers by exposure to high energyionizing radiation at a total dose of from about 0.1 to 15 Mrads, saidcompositions comprising diolefin rubbers having an average molecularweight of from about 100,000 to about 500,000 together with from about0.1 to 5% based on the total weight of at least one polyhydrosulfidedirect radiation vulcanization promoter together with from about 0.1 to5% of an indirect radiation vulcanization promoter which is a lowmolecular weight halogenated aliphatic or aromatic hydrocarboncontaining up to about 10 carbon atoms.
 2. A composition as in claim 1wherein the halogenated hydrocarbon indirect radiation promoter isselected from the group consisting of dichlorobenzenes, dibromobenzenes,difluorobenzenes, bromobenzene, chlorobenzene, chlorotoluenes, mono- andpolychlorinated biphenyls, dichloromethacrylates, chloroform, carbontetrachloride, carbon tetrabromide, hexachloroethane,1,2-dichloro-4-t-butylbenzene, t-butyl chloride, cyclohexyl bromide,a-chlorotoluene, and a, a'-dichloroxylene.
 3. A composition as in claim1 wherein at least one direct promoter is 1,10-dimercaptodecane and atleast one indirect promoter is a dichlorobenzene.
 4. A composition as inclaim 1 wherein the direct promoter is trimethylolpropanedithioglycolate and the indirect promoter is chlorinated biphenyl.
 5. Acomposition as in claim 1 wherein the concentration of polyhydrosulfideis from 0.25 to 1.5%.